Every August e-mails circulate which suggest we are about to have a close encounter with Mars. The e-mails are a hoax, but they say something about our fascination with the Red Planet.

The e-mail seems to promise something truly remarkable.

It often starts: "The Red Planet is about to be spectacular."

It ends with the screaming caps: "NO ONE ALIVE TODAY WILL EVER SEE THIS AGAIN."

The message tells of Mars being close to the earth on 27 August, close enough to be as big as the moon with the naked eye.

Sadly, in three weeks' time, on 27 August, Mars will be a long way away. But between then and now, astronomers will be bombarded with questions by curious punters about this close encounter.

"The e-mails and rumours go out every year but it isn't true. It's something we get asked in the planetarium a lot," says Dr Claire Bretherton, astronomy learning officer at the Royal Observatory Greenwich. "I get e-mails from my friends to me to check whether it's true."

Keith Cooper, editor of Astronomy Now magazine, is also well used to getting a sprinkling of Mars hoax e-mails every year.

Propagation mystery

"It seems to do the rounds all the time. Often there are people asking about it. It's variations on a theme, it isn't the same e-mail each year.

"I'm not sure how it started or who keeps propagating it. But it does seem to be prevalent."

The strange thing about the hoax e-mail is that most of it was true… in 2003.

Then, on 27 August, Mars came within 35 million miles (56 million km) of Earth. That compares very favourably with the next close encounter. At the end of January 2010 it will be 66 million miles (99 million km) away.

"It happens because the earth goes around the Sun in 365 days and Mars goes around in 685 days," says Prof Colin Pillinger, mastermind of the Beagle 2 component of the Mars Express mission. "The Earth's orbit is only very slightly elliptical. Mars is a very elliptical orbit."

Every 26 months there is a close encounter. And every 17 or so years there is a really close encounter.